An acceleration sensor is often used as a sensor for actuating an air bag of an automobile, and detects an impact in a collision of the automobile as acceleration. For the automobile, a one-axis (uniaxial) or two-axis (biaxial) detection function has been enough to measure acceleration on the X-axis and/or the Y-axis. The acceleration to be measured has been very great. Recently, the acceleration sensor has found frequent use in portable terminal equipment and robots, and there has been demand for a three-axis (triaxial) acceleration sensor for measuring accelerations in the X-, Y- and Z-axis directions, in order to detect spatial movements. Also, there has been demand for a high-resolution downsized sensor for detection of micro-acceleration.
Many acceleration sensors adopt a configuration in which the movement of a weight portion or a flexible portion is converted into an electrical signal. Among the acceleration sensors with this configuration are those of the piezoresistor or piezoresistance element type which detect the movement of the weight portion from a change in the resistance of the piezoresistance element provided in the flexible portion coupled to the weight portion, and those of the electrostatic capacity type which detect the movement of the weight portion from a change in electrostatic capacity between the weight portion and a fixed electrode.
Conventional triaxial acceleration sensors shown in Patent Document 1 and Patent Document 2 will be described below. In a triaxial acceleration sensor 101, as shown in FIGS. 11 and 12, a triaxial acceleration sensor element 103, and an IC 104 for control, which performs the amplification, temperature compensation, etc. of a sensor element signal, are laminated and fixed within a ceramic case 102. A cover 105 and the case 102 are joined together to seal up the triaxial acceleration sensor element 103 and the IC 104 within the case 102. As shown in FIG. 12, the triaxial acceleration sensor element 103 is secured to the case 102 with the use of a resin adhesive material 106, and the IC 104 is secured onto the triaxial acceleration sensor element 103 with the use of a resin adhesive material 107.
The triaxial acceleration sensor element 103 has sensor terminals 108, the IC 104 has IC terminals 109, and the case 102 has case terminals 110. The sensor terminals 105 and the IC terminals 109 are interconnected by wires 111, and the IC terminals 109 and the case terminals 110 are interconnected by the same wires 111, so that signals from the sensor are taken outwardly from output terminals 112 interconnected to the case terminals 110 provided in the case 102. The cover 105 is secured to the case 102 by an adhesive material 102a such as a AuSn solder.
In a plan view shown in FIG. 13, the triaxial acceleration sensor element 103 includes a square support frame portion 113, a weight portion 114, and paired beam portions sandwiching the weight portion 114, the weight portion 114 being held in the center of the support frame portion 113 by the two pairs of beam portions 30. Piezoresistance elements are provided in the beam portions 115.
X-axis piezoresistance elements 116 and Z-axis piezoresistance elements 118 are provided in the pair of beam portions 115, and Y-axis piezoresistance elements 117 are provided in the other pair of beam portions 115. The piezoresistance elements are arranged at the four bases of the pair of beam portions 115, and they are interconnected to constitute a bridge circuit. By so doing, uniform resistance changes in the piezoresistance elements are cancelled. By changing the manner of connection of the bridge circuit, moreover, accelerations on the X-axis, the Y-axis and the Z-axis are separated and detected. The sensor terminals 108 are arranged on the support frame portion 113.
The principle of acceleration detection by the bridge circuit will be described by reference to FIGS. 14A to 14D. FIGS. 14A and 14B show the movements of the weight portion 114 when accelerations are applied in the X direction and the Z direction by X-Z planes. When acceleration is applied in the X direction as in FIG. 14A, for example, the weight portion 114 rotates about its site in the vicinity of its upper end center, whereupon the beam portions 115 deform. In accordance with the deformation of the beam portions 115, stress imposed on four X-axis piezoresistance elements X1 to X4 provided on the upper surface of the beam portions 115 changes, and resistance also changes. In this case, X1 and X3 are subjected to tensile stress, while X2 and X4 are placed under compressive stress. As a result, a difference appears in the midpoint potential of a bridge circuit for X-axis detection shown in FIG. 14C, so that an output conformed to the magnitude of acceleration is obtained. When acceleration in the Z direction is applied as shown in FIG. 14B, on the other hand, tensile stress acts on piezoresistance elements Z2, Z3 and compressive stress acts on piezoresistance elements Z1, Z4, with the result that an output is obtained by a bridge circuit for Z-axis detection in FIG. 14D.
The X-axis piezoresistance elements X1 to X4 and the Z-axis piezoresistance elements Z1 to Z4 are formed on the same beam portions 115, but they are different in the configuration of the bridge circuit. Thus, even if the beam portions 115 deform, as in FIG. 14A, in response to the X-direction acceleration, for example, the change in resistance is cancelled in the bridge circuit for Z-axis detection in FIG. 14D, and no change occurs in the output. In this manner, the X-axis acceleration and the Z-axis acceleration can be separated and detected. Detection of the Y-axis acceleration is carried out by the piezoresistance elements formed on the other pair of the beam portions 115 orthogonal to the X-axis, as is done for detection of the X-axis acceleration.
On the other hand, a method for realizing a downsized and inexpensive acceleration sensor by use of a resin protected package technology widely used in a semiconductor mounting technology is known, as shown in Patent Document 3. With this method, a technology for joining covers to the top and bottom of a triaxial acceleration sensor element 103 having movable portions to encapsulate it is used to protect the triaxial acceleration sensor element from a molding resin.
FIG. 15A shows a sectional view of the assembly structure of a triaxial acceleration sensor element having covers joined to the top and bottom thereof by the above-mentioned method, and FIG. 15B shows a plan view of a triaxial acceleration sensor element 120. An upper cover 121 and a lower cover 122 are joined to the top and bottom of the triaxial acceleration sensor element 120 to encapsulate movable portions of the triaxial acceleration sensor element 120 in a closed space. Joining of the triaxial acceleration sensor element 120, the upper cover 121, and the lower cover 122 is carried out by various methods, such as metal bonding or anodic bonding. Here, metal bonding will be shown as an example.
A joining metal region 123 as shown in FIG. 15B is formed on the face and back of the triaxial acceleration sensor element 120. Joining metal regions are also formed in the upper cover 121 and the lower cover 122. They are superposed, pressurized and heated for joining. With this joining step, before the triaxial acceleration sensor elements 120 are taken as individual pieces from a silicon wafer, the silicon wafer having many of the triaxial acceleration sensor elements 120 formed therein, an upper cover silicon wafer having many of the upper covers 122 formed therein with the same pitch, and a lower cover silicon wafer having many of the lower covers 123 formed therein with the same pitch are joined together. This step is called wafer level packaging (hereinafter referred to as WLP). After the closed space is formed by the WLP, the resulting composite is divided into individual chips by dicing. Hereinafter, the individual chip after encapsulation by the WLP will be termed a covered acceleration sensor element 124.
Next, a triaxial acceleration sensor 125 assembled into a package using resin will be described by reference to a sectional view in FIG. 16. An IC 127 for control is fixed onto a lead frame 126 with an adhesive material 128, and the covered acceleration sensor element 124 is fixed onto the IC 127 with an adhesive material 129. Sensor terminals 130 of the covered acceleration sensor element 124 and IC terminals 131 of the IC 127 are connected using wires 132, and the IC terminals 131 and terminals of the lead frame 126 are connected by wires. A structure assembled from the covered acceleration sensor element 124, the IC 127, and the lead frame 126 is molded with a molding resin 133 by use of the transfer mold method. After the resin is cured within, a die, the product is withdrawn from the die to obtain the triaxial acceleration sensor 125. There may be adopted a method in which a plurality of the triaxial acceleration sensors are handled collectively up to the stage of resin molding, released from the die, and then diced to separate them into the individual triaxial acceleration sensors.
With the above-described acceleration sensor obtained using the WLP and resin mold packaging, the movable portions of the triaxial acceleration sensor element 120 can be protected in the silicon wafer stage. Thus, handling in subsequent steps is easy, and does not require strict control over foreign matter. Since the movable portions of the triaxial acceleration sensor element 120 are protected, moreover, the surroundings can be encapsulated by the transfer mold method. In this manner, package assembly can be performed by the resin mold packaging technology, which is often used for conventional IC chips, without the need to use an expensive ceramic package, whereby a small and inexpensive triaxial acceleration sensor can be realized.
The triaxial acceleration sensor 125 shown in FIG. 16, however, poses the following problems in comparison with the triaxial acceleration sensor 101 shown in FIG. 12.
The molding resin and the lead frame used in the triaxial acceleration sensor 125 are different from silicon, which is the material for the covered acceleration sensor element, in the coefficient of thermal expansion. Thus, a temperature change causes thermal stress, exerting external force on the covered acceleration sensor element, thereby changing piezoresistance. Furthermore, when the triaxial acceleration sensor 125 is installed by soldering on a product substrate of a subject product to be mounted with a sensor, the influence of thermal expansion of the product substrate is transmitted to the triaxial acceleration sensor 125 and the covered acceleration sensor element via the soldered region.
With the triaxial acceleration sensor 101 of the ceramic package shown in FIG. 12, the triaxial acceleration sensor element 103 is held in the space within the package. By using a flexible material as the resin 107, therefore, force from the product substrate can be minimally transmitted to the triaxial acceleration sensor element 103.
With the resin-packaged triaxial acceleration sensor 125 shown in FIG. 16, on the other hand, the covered acceleration sensor element 124 has its surroundings covered with the molding resin 133, so that force from the product substrate is apt to be transmitted to the triaxial acceleration sensor element 120. If nonuniform stress changes are caused to the four piezoresistance elements on each axis upon application of external force to the triaxial acceleration sensor element 120, the zero-level of output fluctuates to change the output of the sensor (hereinafter, this zero-level fluctuation will be termed an offset change).
The offset change responsive to the temperature change of the acceleration sensor can be corrected with the IC for detection before the sensor is installed on the product substrate. If the influence of force from the product substrate is exerted during mounting of the product, however, the outcome is produced that the sensor, when installed on the product substrate of various subject products, differs in the characteristics of the change responsive to the temperature.
When the external force from the wiring substrate or the protective package is applied to the covered acceleration sensor element 124, the disposition of the covered acceleration sensor element 124 near the center of the package allows its deformation due to the external force to be nearly bilaterally symmetrical, with the result that outputs on the X-axis and the Y-axis remain unchanged.
However, if a difference occurs between the piezoresistance element near the frame portion (will hereinafter be termed the frame-side piezoresistance element) and the piezoresistance element near the weight portion (will hereinafter be termed the weight-side piezoresistance element), output on the Z-axis changes.
Patent Document 4 describes an acceleration sensor whose output minimally changes under the influence of external force. In this acceleration sensor, stress separation grooves are formed in a frame body to separate it into an outer frame and an inner frame, and both frames are connected by stress relaxation beams having flexibility. The outer frame is connected to a support substrate, and the inner frame is joined to the support substrate by a partial junction. A cover body enclosing the inner frame and a weight portion together with the support substrate and the outer frame is joined to the outer frame. The area of joining of the inner frame to the support substrate is rendered relatively small, and the inner frame is connected to the outer frame by the stress relaxation beams. Thus, even if thermal stress occurs in the outer frame or the support substrate, the inner frame is minimally deformed, so that variations in output can be minimally caused.